This invention relates to a hydraulic apparatus and, more particularly, to a transmission inching control apparatus for industrial vehicles.
There are various conventional methods available for controlling the inching of a transmission for industrial vehicles, particularly a transmission having a torque converter and a hydraulically operated friction engaging element (generally a clutch). In one such manner, as disclosed in the specification of Japanese Patent Publication No. 52-34088 (FIG. 1), an inching valve is used in a hydraulic system composed of a pressure source, a pressure regulating valve, a selector valve, a change valve, a modulator valve, a clutch and the inching valve itself, and clutch pressure is subjected to inching control through controlling the regulating pressure of the modulator valve by the inching valve in accordance with the stroke of the depressed brake pedal. With this manner, the inching valve has no influence upon the modulator valve output pressure when the pedal is held depressed by only a slight degree. When the amount of pedal depression exceeds a predetermined value, a controlling pressure, which from that point onward diminishes in accordance with an increase in the amount of pedal depression, acts upon the clutch as the modulator valve output pressure.
Though the modulator valve is arranged to smoothen the engagement of the clutch, with the above-described method there is still some residual clutch pressure even when the inching valve is in the region of near maximum displacement, as shown in the graph of FIG. 1. This residual pressure causes the clutch to drag when the inching valve has been displaced its full stroke, namely when the brake pedal is held fully depressed. This results not only in a shorter clutch lifetime but also in the danger of a sudden forward movement when the driver backs off the brake pedal slightly under a condition where the residual pressure is large in magnitude.
In another arrangement of the foregoing type, a disadvantage is encountered in that the clutch pressure (inching control pressure) varies with the oil temperature and engine rpm, as shown in the graph of FIG. 2, so that the point at which the clutch starts to engage (or disengage) is not determined precisely in accordance with valve displacement.